This invention relates generally to a method of making a golf ball and, more particularly, to reaction injection and compression molding of a thin layer in a golf ball.
Golf balls are typically comprised of a cover that is compression molded, injection molded, or cast over a golf ball core and which may include one or more wound or solid layers and also a liquid or solid center. Individual layers, including outer core layers, intermediate layers, inner cover layer and outer cover layers are generally either compression or injection molded.
In compression molding, previously molded hemispheres are placed around a core to form a shell. The subassembly is placed between two compression mold halves, which are then heated and pressed together. The shells are often thickest at their deepest point to enhance good surface formation and evacuation of entrapped gases. Compression molding does not require support member for the core or other components for adding materials. Details on the compression molded product, such as dimples, are in general significantly sharper than that of injection molding. However, compression molding does necessitate pre-production of the molded half shells around the core, and pre-alignment of the shells within the mold halves by hand or by machine. Furthermore, weak points form at the parting line where the material of the two shells melt and flow together.
Injection molding is generally conducted between two mold halves that together define a mold cavity in which the core is supported with fixed or retractable pins. Cover material is melted at high temperature and injected at high pressures into the mold cavity and around the core. The preferred retractable pins are withdrawn when the cover material is solid enough to support the core, yet soft enough to fill the remaining space left by the pins. Injection molding methods are typically conducted with the mold closed, with plastic pressures upwards of about 12,000 psi. These high pressures tend to deform the golf ball core through compression. Also, equipment for injection molding typically includes extremely small air vents, which significantly limit the injection speed of the cover or layer material.
Generally, multiple ports around the core are used to inject the cover or layer material to speed the injection process. Where material flowing from the various ports meet, weld lines, or knit lines, can be formed, resulting in discontinuities and residual stresses across the weld lines. This results in poor finishes with poor definition of features such as dimples that are molded into the layer or cover. Golf ball material failures also tend to occur at the weld lines after repeated use of the golf ball. Due to friction between the injected material and the mold cavity and core, the material catches at various locations on the mold halves and core during its injection, creating more internal stresses and further decreasing the homogeneity of the molded material. These stresses also tend to produce lower quality finishes and areas that are more prone to structural failure.
As with the above-referenced processes, a casting process also utilizes pairs of mold cavities. In a casting process, a cover material, typically a thermoset polyurethane, is introduced into a first mold cavity of each pair. Then, a core is held in position (e.g. by an overhanging vacuum or suction apparatus) to contact the cover material in what will be the spherical center of the mold cavity pair. Once the cover material is at least partially cured (e.g., a point where the core will not substantially move), the core is released, the cover material is introduced into a second mold cavity of each pair, and the mold is closed. The closed mold is then subjected to heat and pressure to cure the cover material thereby forming a cover on the core. With injection molding, compression molding, and casting, the molding cavities typically include a negative dimple pattern to impart a dimple pattern on the cover during the molding process.
Recently, a particular form of injection molding, reaction injection molding (xe2x80x9cRIMxe2x80x9d), has been receiving increased attention, particularly as a method of forming a polyurethane or polyurea based cover or layer in a golf ball. RIN is a process by which highly reactive liquid components are injected into a closed mold, mixed usually by impingement and/or mechanical mixing in an in-line device such as a xe2x80x9cpeanut mixer,xe2x80x9d where they polymerize primarily in the mold to form a coherent, one-piece molded article. The RIM process usually involves a rapid reaction between the reactive liquids, often in the presence of a catalyst. The liquids are stored in separate tanks, preheated to about 90xc2x0 F. to 150xc2x0 F., metered in the desired weight to weight ratio and fed into an impingement mix head, with mixing occurring under high pressure, e.g., 1,500 to 3,000 psi. The material is then injected into the mold, in where the liquids react rapidly to gel and form polymers such as polyurethanes, polyurea, epoxies, and various unsaturated polyesters. Both the mix head and the mold are heated to ensure proper injection viscosity of the material.
Because RIM involves a chemical reaction that transforms liquid monomers and/or adducts into polymers, its mold need not be made to withstand the high temperatures and high pressures in conventional injection molding. Plus, the RIM process is fast. The chemical reaction causes the material to set in less than one minute and in many cases in about 10 seconds or less. However, the close mold design in conventional RIM limits the thickness of the molded cover or layer to be no less than about 0.02 inches. Thinner covers and layers in golf ball are preferred for various reasons. Ultra-thin layers can provide a transition between a soft outer cover layer and a hard inner cover layer, providing a means to tune the golf ball""s spin rate profile for medium to short iron play. Alternatively, as an inner cover layer, an ultra-thin layer can reduce driver spin.
Therefore, it is desirable to provide a method for molding ultra-thin layers in golf balls that combine the advantages of reaction injection molding and compression molding, while eliminating their shortcomings.
The present invention is directed to a method of making golf balls and, in particular, a method of molding a thin layer of a thermoset or thermoplastic material around a golf ball component. The thin layer may be continuous or discontinuous. In a preferred embodiment, the method involves a mold having a mold cavity defined by opposing mold potions. The golf ball component is centered within the mold cavity by a plurality of support members such as retractable pins, and the mold portions are held together with a first tonnage, leaving a cavity space between the component and the mold portions.
A pre-mixed reaction mixture is injected into the cavity space under a first tonnage. The mixture comprises an isocyanate and a polyol or polyamine, with one or more optional additives such as curing agents, crosslinking agents, catalysts, fillers, accelerators, processing aids, processing oils, plasticizers, foaming agents, colorants, or radical quenchers. The pressurized mixture overcomes the first tonnage, pushes the mold portions apart from each other to a partially open position with an opening therebetween, and fills the cavity space. Preferably the opening between the mold portions is at least about 0.001 inches; more preferably, it ranges from about 0.001 inches to about 0.015 inches.
After a sufficient amount of the mixture has been injected to maintain the component substantially in the predetermined position, the support members are removed. As the reaction mixture fills the cavity space, the reactants react partially within the cavity space. After at least about 80 percent of the cavity space is injected with the reaction mixture, a second and higher tonnage is provided that is sufficient to overcome the hydraulic pressure of the reaction mixture and compress the mold portions toward each other to the closed position to compression mold the thin layer. Preferably, the second tonnage is at least about 20 percent greater than the first tonnage. Alternatively, the hydraulic pressure is removed in the compression molding phase so that the first tonnage is sufficient to compress the mold portions and close them back together.
While in the cavity space, the reaction mixture completes its reaction and is cooled to a temperature below its freezing point to form the thin layer of the thermoset or themoplastic material immediately about the golf ball component. Preferably the thin layer has a thickness of no greater than about 0.03 inches, more preferably from about 0.0005 inches to about 0.02 inches, and most preferably from about 0.001 inches to about 0.01 inches.
An alternative method of molding the thin layer starts with the opposing mold portions held apart with the opening of at least about 0.001 inches therebetween. The reactants are preheated and mixed to form the reaction mixture, which is then injected into the cavity space at a plurality of locations and allowed to partially react. Compression molding is initiated preferably after at least about 80 percent of the cavity space is filled with the reaction mixture. The mold portions are compressed toward each other to a closed position under a tonnage. The thermoset or thermoplastic material formed form the mixture substantially encapsulates the component with a layer of from about 0.0005 inches to about 0.02 inches thick.
In a further embodiment of the present invention, a method of molding the thin layer involves injecting a pre-mixed reaction mixture of at least two reactants under a hydraulic pressure into the cavity space through an opening of at least about 0.001 inches between the mold portions to surround the component, filling at least about 80% of the cavity space with the mixture. Compression molding ensues after the mixture is partially reacted, by using a tonnage to close the mold portions together. The reaction mixture completes the reaction and forms the thin layer of a thermoset or thermoplastic material around the component no greater than about 0.03 inches in thickness.